Latrine Logger Unit

Water For People is an international non-profit working across 9 countries to bring safe water and sanitation to 4 million people.

Water for People want to see communities break free from the cycle of poverty and spend time growing, learning, and thriving, instead of walking for water and fighting off illness.

Following on from testing a prototype unit, we have done many improvements and have supplied the Latrine Logger – a data logging unit for monitoring pit latrines.

Problem

1.8 billion people around the world don’t have access to safe water and 2.4 billion lack access to adequate sanitation. Women and children spend more than 4 hours walking for water each day, and more than 840,000 people die each year from water-related diseases. (from Water for People website)

Water For People is an international non-profit working across 9 countries to bring safe water and sanitation to 4 million people.

Water for People want to see communities break free from the cycle of poverty and spend time growing, learning, and thriving, instead of walking for water and fighting off illness.

Water for People have been installing pit latrines in various different areas in Uganda. These are filling up at very different rates, but the reason for this is unknown. They need to record the usage of the latrines with a simple, low-cost, robust and non-intrusive system.

Overview

The Latrine Logger uses a capacitative sensor to measure liquid flow down non-conductive pipes, such as the PVC pipes generally used for pit latrines. This sensor is wrapped around the PVC pipe and the capacitance forms part of the RC circuit in a 555 timer oscillator. This gives an output frequency of around 60kHz which will drop when any liquid is sent down the pipe. This drop is monitored and, along with filtering and averaging algorithms, triggers the unit to record a ‘flush event’.

The unit records each flush event with a time and date stamp into a .csv file onto an on-board SD card. This data is human readable and can be opened in any spreadsheet program.

The unit is battery powered by 3 x ‘D’ size cells, which should last for between 1-2 months.

There is an on-board real time clock used for the timestamp.

We are using an ATMEGA328 microprocessor with the Arduino bootloader and code has been written for upload via the Arduino IDE.

The prototype was developed along with Nicola Greene (in conjunction with Water for People) and James Fowkes (a fellow Nottingham Hackspace member) to look at the issue of data monitoring for pit latrines. The prototype units were used in Uganda and feedback from their operation was fed back to improving the unit.

Flush Sensor

We needed an easy to install sensor to fit over the PVC flush pipe. We came up with a simple wrap-around sensor comprised of two pieces self-adhesive aluminium tape. These are stuck to a plastic backing with one wire from each of the two pieces of tape. These each wrap half way around the PVC pipe and form a very small value capacitor. This capacitance varies when water passes through the PVC pipe.

To measure the capacitance we use the capacitor to form an astable oscillator using a standard 555 timer circuit. This produces a frequency which is proportional to the capacitance. Even small amounts of water change the capacitance to trigger the logger.

Schematic

Instructions

Data analysis

The flush sense algorithm monitors the average frequency from the 555 timer circuit. A flush start is detected when the frequency drops by a certain amount and brings the frequency below the average value by a certain amount (in this case its around a 1000Hz drop). The flush end is detected when the frequency has risen above the low value by another amount (in this case around 500Hz). This is best shown by the digrams here:

This graph shows the drop in frequency when water is flushed down the pipe. There is a sharp drop then a slow rise as the water trickles down.

The frequency must drop below the average frequency minus a threshold value before the unit registers the start of a flush event.

The frequency must rise above the end threshold (the average minus a different threshold value) to register the end of a flush event.

The files are stored onto an SD card (size from 1Gb to 8Gb). The data is written into a .csv file withthe ID of the unit, the timestamp and the event. This also measures the battery voltage and when the battery voltage is low then ‘BATT LOW’ is written to the card. Any data after this point is not reliable and the unit will stop recording data.

Design files

This project is open-source and all the design files are available here. This includes the embedded software code, the schematic and the PCB design files.

Enquiries

Re-Innovation

Renewable Energy Innovation specialise in electrical and electronic systems for renewable energy projects, mainly solar, wind and micro-hydro. We focus on renewable energy based stand-alone power supply systems (off-grid systems). This includes power and energy monitoring, battery charge control and wiring systems. Please contact us to talk about your project. If you require consultancy, design and implementation services for your renewable energy project please contact us.

Curious Electric Company

The Curious Electric Company specialise in creating electronic kits that help you monitor and measure the world around you, with clever data loggers and sensors you can take the pulse of your planet, collect and share data and knowledge with the world. Be informed - stay curious!

Bespoke Gear

Our sister business, Bespoke Gear have been building interactive pedal-power systems for over 15 years. Doing events and selling equipment to a wide variety of customers in that time we have learnt what works. We design and build high-quality, robust pedal-powered equipment and bespoke, interesting and interactive displays. If you are trying to promote environmental awareness, energy efficiency, getting people fit or just want something thats a bit different for your event then we might have something for you.